//g++ -std=c++11 -Wall -O2 -o main.exe main.cpp
//g++ -std=c++14 -Wall -O2 -o main.exe main.cpp

#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <iostream>
#include <iomanip>
#include <string>
#include <vector>
#include <map>
#include <queue>
#include <stack>
#include <tuple>
#include <functional>
#include <algorithm>
#include <complex>
#include <numeric>

using namespace std;

#define spc " "
#define MOD 1000000007

typedef long long ll;
typedef long double ld;
typedef pair<int, int> p_ii;
typedef tuple<int, int, int, int> tup;

bool tupComp(tup &t1, tup &t2){
    return get<2>(t1) > get<2>(t2);
}

//------------------------------------------------------------------------------

int main(){
    string s;
    cin >> s;

    int c = 0, ans = 0;
    map<int, int> m;
    m[0] = 0;
    for(int i = 0; i < s.size(); i++){
        c += (s[i] == 'A' ? 1 : -1);
        
        if(c != 0 && m[c] == 0) m[c] = i + 1;
        else{
            ans = max(ans, i + 1 - m[c]);
        }
    }
    cout << ans << endl;

    return 0;
}